For converting fuel energy into heat, so-called impulse or high-velocity burners are increasingly used. The fuel and the combustion air are mixed with one another and ignited in a high-heat-resistant combustion chamber, which is typically made of ceramic material. The resultant hot combustion gases flow at high velocity through a nozzle-shaped outlet into the heating chamber, which is formed either by the furnace chamber itself or by the interior of radiator heating tube. The outlet of the combustion chamber can be constricted in nozzle-like fashion or formed by a nozzle ring; the axes of the nozzle openings may be parallel to one another or may be oriented divergently from one another. The mechanical energy of the gas stream or jet emerging from the combustion chamber, which derives predominantly from the fuel, serves to mix and circulate the gases in the heating chamber, which in turn promotes the temperature equalization in a desired manner. As the development of ceramic material for heavy-duty combustion chambers progresses, impulse burners today are employed even at process temperatures above 800.degree. C.; as a rule, the combustion air is preheated to improve the efficiency, and this is done for example in a preceding recuperator, often in the form of a tubular ribbed recuperator.
A low-pollution industrial burner with this basic design is known from U.S. Pat. No. 4,586,894, claiming the priority of German Patent 34 22 229. The thermal nitrogen oxide (NO.sub.x) formation is reduced by intensive mixing of the flame gases with relatively cold furnace gases (known as internal recirculation), and the effect is increased by supplying air in graduated fashion, as described in detail in this reference. Despite these provisions, in burners of this type, with air preheating of 600.degree. C., NO.sub.x emissions can rise above the 200 ppm mark, which is thus in the vicinity of the legal limit.
A further lowering of NO.sub.x emissions is enabled, conversely, by a more recent impulse burner (U.S. Pat. No. 5,154,599, claiming the priority of European Patent 0 463 218), in which once the ignition temperature in the heating chamber is reached, the combustion air is switched over entirely to an external nozzle ring, and consequently no further combustion occurs in the combustion chamber. For a specified nozzle geometry, extremely low NO.sub.x values can then be attained (below 10 ppm). With externally preheated air, however, two hot air valves are needed in this burner, while in the version as a recuperator and regenerator burner, the combustion air, in the heating-up mode, does not flow via the heat exchangers.
A burner with reduced NO.sub.x emissions and with a switchover of fuel delivery to convert the burner from the cold startup mode to the normal operating mode is known from European Patent EP-A1 0 343 746. This burner does not employ a combustion chamber; instead, the arrangement provides air and fuel supply means that discharge at a certain lateral distance from the inner wall of the furnace. Until the ignition temperature is reached, or in other words in the startup mode, the fuel is introduced into the air supply line at a distance from its mouth; once the operating temperature in the furnace chamber is reached, the switchover is made to fuel nozzles, which are located at a predetermined distance laterally from the mouth of the air supply line (EN) in the inner wall of the furnace. According to the specification and drawing, this burner has no characteristics are a prerequisite for a impulse burner, namely a nozzle outlet for the combustion air, which makes up the primary component of the mass flow. Even during the startup mode, or in other words upon heating up to the ignition temperature, combustion already takes place essentially in the furnace chamber. The burner is not arranged for operation with an integrated recuperator or regenerator.